Development of 18F-labeled picolinamide probes for PET imaging of malignant melanoma

Melanin-Targeting Radiotracers and Their Preclinical, Translational, and Clinical Status: From Past to Future

Melanin is one of the representative biomarkers of malignant melanoma and a potential target for diagnosis and therapy. With advancements in chemistry and radiolabeling technologies, promising strides have been made to synthesize radiolabeled melanin-binding molecules for various applications. We present an overview of melanin-targeted radiolabeled molecules and compare their features reported in preclinical studies. Clinical practice and trials are also discussed to elaborate on the safety and validity of the probes, and expanded applications beyond melanoma are reviewed. Melanin-targeted imaging holds potential value in the diagnosis, staging, and prognostic assessment of melanoma and other applications. Melanin-targeted radionuclide therapy possesses immense potential but requires more clinical validation. Furthermore, an intriguing avenue for future research involves expanding the application scope of melanin-targeted probes and exploring their value.

Synthesis and evaluation of 18F-labeled procainamide as a PET imaging agent for malignant melanoma

Malignant melanoma has an aggressive nature and a high metastatic propensity resulting in the highest mortality rate of any skin cancer. In this study, we synthesized 18F-labeled procainamide (PCA) for detection of melanoma using positron emission tomography (PET), and evaluated its biological characteristics. The non-decay-corrected radiochemical yield of 18F-PCA was 10-15% and its in vitro stability was over 98% for 2 h. At 1 h, cellular uptake of 18F-PCA was 3.8-fold higher in a group with the presence of l-tyrosine than in a non-l-tyrosine-treated group. Furthermore, 18F-PCA permitted visualization of B16F10 (mouse melanoma) xenografts on microPET after intravenous injection, and was retained in the tumor for 60 min, with a high tumor-to-liver uptake ratio. 18F-PCA showed specific melanoma uptake in primary lesions with a high melanin targeting ability in small animal models. 18F-PCA may have potential as a PET imaging agent for direct melanoma detection.

Highlights on U.S. FDA-approved fluorinated drugs over the past five years (2018-2022)

The objective of this review is to provide an update on the fluorine-containing drugs approved by U.S. Food and Drug Administration in the span of past five years (2018-2022). The agency accepted a total of fifty-eight fluorinated entities to diagnose, mitigate and treat a plethora of diseases. Among them, thirty drugs are for therapy of various types of cancers, twelve for infectious diseases, eleven for CNS disorders, and six for some other diseases. These are categorized and briefly discussed based on their therapeutic areas. In addition, this review gives a glimpse about their trade name, date of approval, active ingredients, company developers, indications, and drug mechanisms. We anticipate that this review may inspire the drug discovery and medicinal chemistry community in both industrial and academic settings to explore the fluorinated molecules leading to the discovery of new drugs in the near future.

Recent Developments in PET and SPECT Radiotracers as Radiopharmaceuticals for Hypoxia Tumors

Hypoxia, a deficiency in the levels of oxygen, is a common feature of most solid tumors and induces many characteristics of cancer. Hypoxia is associated with metastases and strong resistance to radio- and chemotherapy, and can decrease the accuracy of cancer prognosis. Non-invasive imaging methods such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) using hypoxia-targeting radiopharmaceuticals have been used for the detection and therapy of tumor hypoxia. Nitroimidazoles are bioreducible moieties that can be selectively reduced under hypoxic conditions covalently bind to intracellular macromolecules, and are trapped within hypoxic cells and tissues. Recently, there has been a strong motivation to develop PET and SPECT radiotracers as radiopharmaceuticals containing nitroimidazole moieties for the visualization and treatment of hypoxic tumors. In this review, we summarize the development of some novel PET and SPECT radiotracers as radiopharmaceuticals containing nitroimidazoles, as well as their physicochemical properties, in vitro cellular uptake values, in vivo biodistribution, and PET/SPECT imaging results.

Recent Advances of 68Ga-Labeled PET Radiotracers with Nitroimidazole in the Diagnosis of Hypoxia Tumors

Positron emission tomography (PET) is a noninvasive molecular imaging method extensively applied in the detection and treatment of various diseases. Hypoxia is a common phenomenon found in most solid tumors. Nitroimidazole is a group of bioreducible pharmacophores that selectively accumulate in hypoxic regions of the body. Over the past few decades, many scientists have reported the use of radiopharmaceuticals containing nitroimidazole for the detection of hypoxic tumors. Gallium-68, a positron-emitting radioisotope, has a favorable half-life time of 68 min and can be conveniently produced by ⁶⁸Ge/⁶⁸Ga generators. Recently, there has been significant progress in the preparation of novel ⁶⁸Ga-labeled complexes bearing nitroimidazole moieties for the diagnosis of hypoxia. This review provides a comprehensive overview of the current status of developing ⁶⁸Ga-labeled radiopharmaceuticals with nitroimidazole moieties, their pharmacokinetics, and in vitro and in vivo studies, as well as PET imaging studies for hypoxic tumors.

MC1R and melanin-based molecular probes for theranostic of melanoma and beyond

Malignant melanoma is accounting for most of skin cancer-associated mortality. The incidence of melanoma increased every year worldwide especially in western countries. Treatment efficiency is highly related to the stage of melanoma. Therefore, accurate staging and restaging play a pivotal role in the management of melanoma patients. Though 18F-fluorodeoxyglucose (18F-FDG) positron-emission tomography (PET) has been widely used in imaging of tumor metastases, novel radioactive probes for specific targeted imaging of both primary and metastasized melanoma are still desired. Melanocortin receptor 1 (MC1R) and melanin are two promising biomarkers specifically for melanoma, and numerous research groups including us have been actively developing a plethora of radioactive probes based on targeting of MC1R or melanin for over two decades. In this review, some of the MC1R-targeted tracers and melanin-associated molecular imaging probes developed in our research and others have been briefly summarized, and it provides a quick glance of melanoma-targeted probe design and may contribute to further developing novel molecular probes for cancer theranostics.

A Mitochondria-Targeted Phenylbutyric Acid Prodrug Confers Drastically Improved Anticancer Activities

Phenylbutyric acid (PBA) has been reported as a dual inhibitor of pyruvate dehydrogenase kinases (PDKs) and histone deacetylases (HDACs), exhibiting anticancer effects. However, the low membrane permeability and poor cellular uptake limit its access to the target organelle, resulting in weak potencies against the intended targets. Herein, we report the design and identification of a novel 4-CF₃-phenyl triphenylphosphonium-based PBA conjugate (53) with improved in vitro and in vivo anticancer activities. Compound 53 exhibited an IC₅₀ value of 2.22 μM against A375 cells, outperforming the parent drug PBA by about 4000-fold. In the A375 cell-derived xenograft mouse model, 53 reduced the tumor growth by 76% at a dose of 40 mg/kg, while PBA only reduced the tumor growth by 10% at a dose of 80 mg/kg. On the basis of these results, 53 may be considered for further preclinical evaluations for cancer therapy.

Application of Self-Assembly Nanoparticles Based on DVDMS for Fenton-Like Ion Delivery and Enhanced Sonodynamic Therapy

Upon harnessing low-intensity ultrasound to activate sonosensitizers, sonodynamic therapy (SDT) induces cancer cell death through the reactive oxygen species (ROS) mediated pathway. Compared with photodynamic therapy (PDT), SDT possesses numerous advantages, including deeper tissue penetration, higher accuracy, fewer side effects, and better patient compliance. Sinoporphyrin sodium (DVDMS), a sonosensitizer approved by the FDA, has drawn abundant attention in clinical research, but there are some deficiencies. In order to further improve the efficiency of DVDMS, many studies have applied self-assembly nanotechnology to modify it. Furthermore, the combined applications of SDT/chemodynamic therapy (CDT) have become a research hotspot in tumor therapy. Therefore, we explored the self-assembly of nanoparticles based on DVDMS and copper to combine SDT and CDT. A cost-effective sonosensitizer was synthesized by dropping CuCl₂ into the DVDMS solution with the assistance of PVP. The results revealed that the nanostructures could exert excellent treatment effects on tumor therapy and perform well for PET imaging, indicating the potential for cancer theranostics. In vitro and in vivo experiments showed that the nanoparticles have outstanding biocompatibility, higher ROS production efficiency, and antitumor efficacy. We believe this design can represent a simple approach to combining SDT and CDT with potential applications in clinical treatment and PET imaging.

Two Decades of Research in Artificial Pancreas: Insights from a Bibliometric Analysis

BACKGROUND

Artificial pancreas is a well-known research topic devoted to achieving better glycemic outcomes that has been attracting increasing attention over the years. However, there is a lack of systematic, chronological, and synthesizing studies that show the background of the knowledge generation in this field. This study implements a bibliometric analysis to recognize the main documents, type of publications, research categories, countries, keywords, organizations, and authors related to this topic.

METHODS

Web of Science core collection database was accessed from 2000 to 2020 in order to select high-quality scientific documents based on a specific search query. Bibexcel, MS Excel, Power BI, R-Studio, VOSviewer, and CorText software were used for a descriptive and network analysis based on the local database obtained. Bibliometric parameters as the h-index, frequencies, co-authorship and co-ocurrences were computed.

RESULTS

A total of 756 documents were included that show a growing scientific production on this topic with an increasing contribution from engineering. Outstanding authors, organizations, and countries were identified. An analysis of trends in research was conducted according to the scientific categories of the Web of Science database to identify the main research interests of the last 2 decades and the emerging areas with greater prominence in the coming years. A keyword network analysis allowed to identify the main stages in the development of the AP research over time.

CONCLUSIONS

Results reveal a comprehensive background of the knowledge generation for the AP topic during the last 2 decades, which has been strengthened with international collaborations and a remarkable interdisciplinarity between endocrinology and engineering, giving rise to a growing number of research areas over time, where computer science and medical informatics stand out as the main emerging research areas.

Development of Radiofluorinated Nicotinamide/Picolinamide Derivatives as Diagnostic Probes for the Detection of Melanoma

Regarding the increased incidence and high mortality rate of malignant melanoma, practical early-detection methods are essential to improve patients’ clinical outcomes. In this study, we successfully prepared novel picolinamide–benzamide (¹⁸F-FPABZA) and nicotinamide–benzamide (¹⁸F-FNABZA) conjugates and determined their biological characteristics. The radiochemical yields of ¹⁸F-FPABZA and ¹⁸F-FNABZA were 26 ± 5% and 1 ± 0.5%, respectively. ¹⁸F-FPABZA was more lipophilic (log P = 1.48) than ¹⁸F-FNABZA (log P = 0.68). The cellular uptake of ¹⁸F-FPABZA in melanotic B16F10 cells was relatively higher than that of ¹⁸F-FNABZA at 15 min post-incubation. However, both radiotracers did not retain in amelanotic A375 cells. The tumor-to-muscle ratios of ¹⁸F-FPABZA-injected B16F10 tumor-bearing mice increased from 7.6 ± 0.4 at 15 min post-injection (p.i.) to 27.5 ± 16.6 at 3 h p.i., while those administered with ¹⁸F-FNABZA did not show a similarly dramatic increase throughout the experimental period. The results obtained from biodistribution studies were consistent with those derived from microPET imaging. This study demonstrated that ¹⁸F-FPABZA is a promising melanin-targeting positron emission tomography (PET) probe for melanotic melanoma.

PET Imaging of Melanoma Using Melanin-Targeted Probe

Melanin exists in the most of melanoma lesions. Melanin plays an important role in melanoma progression, metastasis, therapy response, and the overall survival of patients. Therefore, melanin is a critical target for melanoma diagnosis and therapy. Many melanin targeting probes, such as radioisotope-labeled benzamide analogs, have been developed for melanoma diagnosis using positron emission tomography (PET). The N-(2-(diethylamino)-ethyl)-¹⁸F-5-fluoropicolinamide (¹⁸F-P3BZA) probe is one of the benzamide analogs and has been preliminarily tested for clinical diagnosis of melanoma in our recent studies. It has shown high specificity and favorable in vivo performance for PET of melanoma. Herein, we describe the detailed synthesis protocol of ¹⁸F-P3BZA and PET/CT imaging procedure for animal models and patients.

Evaluation of Radioiodinated Fluoronicotinamide/Fluoropicolinamide-Benzamide Derivatives as Theranostic Agents for Melanoma

Malignant melanoma is the most harmful type of skin cancer and its incidence has increased in this past decade. Early diagnosis and treatment are urgently desired. In this study, we conjugated picolinamide/nicotinamide with the pharmacophore of ¹³¹I-MIP-1145 to develop ¹³¹I-iodofluoropicolinamide benzamide (¹³¹I-IFPABZA) and ¹³¹I-iodofluoronicotiamide benzamide (¹³¹I-IFNABZA) with acceptable radiochemical yield (40 ± 5%) and high radiochemical purity (>98%). We also presented their biological characteristics in melanoma-bearing mouse models. ¹³¹I-IFPABZA (Log P = 2.01) was more lipophilic than ¹³¹I-IFNABZA (Log P = 1.49). B16F10-bearing mice injected with ¹³¹I-IFNABZA exhibited higher tumor-to-muscle ratio (T/M) than those administered with ¹³¹I-IFPABZA in planar γ-imaging and biodistribution studies. However, the imaging of ¹³¹I-IFNABZA- and ¹³¹I-IFPABZA-injected mice only showed marginal tumor uptake in A375 amelanotic melanoma-bearing mice throughout the experiment period, indicating the high binding affinity of these two radiotracers to melanin. Comparing the radiation-absorbed dose of ¹³¹I-IFNABZA with the melanin-targeted agents reported in the literature, ¹³¹I-IFNABZA exerts lower doses to normal tissues on the basis of similar tumor dose. Based on the in vitro and in vivo studies, we clearly demonstrated the potential of using ¹³¹I-IFNABZA as a theranostic agent against melanoma.

Ultrasensitive detection of malignant melanoma using PET molecular imaging probes

With the emergence of new therapeutic modalities, the diagnosis of melanoma at the earliest practicable stage has become more important for improving the survival of patients. We developed a positron emission tomography (PET) imaging probe, N-(2-(dimethylamino)ethyl)-5-[¹⁸F]fluoropicolinamide ([¹⁸F]DMPY2) and evaluated diagnostic performance in animal models. [¹⁸F]DMPY2 PET exhibited excellent performance in detecting primary and metastatic melanomas, demonstrating strong/prolonged tumoral uptake and rapid background clearance. This suggests that this radiotracer could be used as a novel PET imaging agent to obtain outstanding image quality in the diagnosis of melanoma. This is the pioneering report of pyridine-based benzamide derivative with reduced alkyl chains in the amine residue and ultrasensitive detection of melanoma lesions in living subjects compared to conventional PET imaging agents.

Malignant melanoma has one of the highest mortality rates of any cancer because of its aggressive nature and high metastatic potential. Clinical staging of the disease at the time of diagnosis is very important for the prognosis and outcome of melanoma treatment. In this study, we designed and synthesized the ¹⁸F-labeled pyridine-based benzamide derivatives N-(2-(dimethylamino)ethyl)-5-[¹⁸F]fluoropicolinamide ([¹⁸F]DMPY2) and N-(2-(dimethylamino)ethyl)-6-[¹⁸F]fluoronicotinamide ([¹⁸F]DMPY3) to detect primary and metastatic melanoma at an early stage and evaluated their performance in this task. [¹⁸F]DMPY2 and [¹⁸F]DMPY3 were synthesized by direct radiofluorination of the bromo precursor, and radiochemical yields were ∼15–20%. Cell uptakes of [¹⁸F]DMPY2 and [¹⁸F]DMPY3 were >103-fold and 18-fold higher, respectively, in B16F10 (mouse melanoma) cells than in negative control cells. Biodistribution studies revealed strong tumor uptake and retention of [¹⁸F]DMPY2 (24.8% injected dose per gram of tissue [ID/g] at 60 min) and [¹⁸F]DMPY3 (11.7%ID/g at 60 min) in B16F10 xenografts. MicroPET imaging of both agents demonstrated strong tumoral uptake/retention and rapid washout, resulting in excellent tumor-to-background contrast in B16F10 xenografts. In particular, [¹⁸F]DMPY2 clearly visualized almost all metastatic lesions in lung and lymph nodes, with excellent image quality. [¹⁸F]DMPY2 demonstrated a significantly higher tumor-to-liver ratio than [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) and the previously reported benzamide tracers N-[2-(diethylamino)-ethyl]-5-[¹⁸F]fluoropicolinamide ([¹⁸F]P3BZA) and N-[2-(diethylamino)-ethyl]-4-[¹⁸F]fluorobenzamide ([¹⁸F]FBZA) in B16F10-bearing or SK-MEL-3 (human melanoma)-bearing mice. In conclusion, [¹⁸F]DMPY2 might have strong potential for the diagnosis of early stage primary and metastatic melanoma using positron emission tomography (PET).

Synthesis of N-(6-[18F]Fluoropyridin-3-yl)glycine as a potential renal PET agent

OBJECTIVE

Given the requirements of high sensitivity and spatial resolution, the development of new positron emission tomography (PET) agents is required for PET renography. The objective of this study was to investigate a new fluorine-18 labeled hippurate analogue of picolinamide, N-(6-[¹⁸F]Fluoropyridin-3-yl)glycine, as a new renal PET agent for evaluating renal function.

METHODS

N-(6-[¹⁸F]Fluoropyridin-3-yl)glycine was prepared via a two-step reaction, including the nucleophilic substitution reaction of Br with ¹⁸F using methyl 2-(6-bromonicotinamido)acetate as a precursor followed the hydrolysis with sodium hydroxide and purification by preparative-HPLC. The in vitro and in vivo stability were determined using HPLC, and the plasma protein binding (PPB) and erythrocyte uptake of N-(6-[¹⁸F]Fluoropyridin-3-yl)glycine were determined using blood collected from healthy rats at 5 min post-injection. Biodistribution and dynamic micro-PET/CT imaging studies were conducted in healthy rats.

RESULTS

N-(6-[¹⁸F]Fluoropyridin-3-yl)glycine was prepared within 45 min with an uncorrected radiochemical yield of 24.5 ± 6.7% (n = 6, based on [¹⁸F]F^-) and a radiochemical purity of >98%. N-(6-[¹⁸F]Fluoropyridin-3-yl)glycine demonstrated good stability both in vitro and in vivo. The results of the biodistribution and dynamic micro-PET/CT imaging studies in normal rats indicated that N-(6-[¹⁸F]Fluoropyridin-3-yl)glycine was rapidly and exclusively excreted via the renal-urinary pathway.

CONCLUSION

N-(6-[¹⁸F]Fluoropyridin-3-yl)glycine is has been shown to be a promising renal PET agent and warrants further evaluation of renal function.

The first radiosynthesis of 2-amino-5-[18F]fluoropyridines via a "minimalist" radiofluorination/palladium-catalyzed amination sequence from anisyl(2-bromopyridinyl)iodonium triflate

The synthesis of 2-amino-5-[¹⁸F]fluoropyridines was achieved in 8-85% yields by palladium-catalyzed reaction of 2-bromo-5-[¹⁸F]fluoropyridine with piperidine, dimethylamine, butylamine, methylpiperazine, benzylamine, aniline and 3-aminopyridine. 2-Bromo-5-[¹⁸F]fluoropyridine was obtained by radiofluorination of anisyl(2-bromopyridinyl-5)iodonium triflate (88% yield). The radiofluorination step was performed under "minimalist" conditions to guarantee a successful subsequent amination reaction.

MnCl2-Catalyzed C-H Alkylation on Azine Heterocycles

Low-valent manganese-catalyzed C-H alkylation of pyridine derivatives with both primary and challenging secondary alkyl halides was established by amide assistance. The strategy provided expedient access to alkylated pyridines with wide functional group tolerance and ample scope through weak chelation. Mechanistic studies provided strong support for a rate-determining C-H activation and a SET-type C-X scission.

N-(2-(Dimethylamino)Ethyl)-4-18F-Fluorobenzamide: A Novel Molecular Probe for High-Contrast PET Imaging of Malignant Melanoma

Malignant melanoma is an aggressive and serious form of skin cancer, with prognosis and treatment outcome depending heavily on the clinical stage of the disease at the time of diagnosis. Here, we synthesized a novel ¹⁸F-labeled benzamide derivative to target melanoma and then evaluated its biologic characteristics in small-animal models. Methods: N-(2-(dimethylamino)ethyl)-4-¹⁸F-fluorobenzamide (¹⁸F-DMFB) was synthesized by reaction of N-succinimidyl 4-¹⁸F-fluorobenzoate with N,N-dimethylethylenediamine. The binding affinity of ¹⁸F-DMFB was measured in B16F10 (mouse melanoma) cells with or without l-tyrosine. Small-animal PET imaging with ¹⁸F-DMFB was performed on B16F10 xenograft and metastasis mouse models. Results: The overall non-decay-corrected radiochemical yield of ¹⁸F-DMFB was approximately 10%-15%. Uptake of ¹⁸F-DMFB was melanin-specific, as cellular uptake in B16F10 increased more than 18-fold in the presence of l-tyrosine. Biodistribution studies revealed that ¹⁸F-DMFB accumulated, and was retained, in B16F10 xenografts for 120 min (10, 30, 60, and 120 min: 9.24, 10.80, 13.0, and 10.59 percentage injected dose/g, respectively) after radiotracer injection. Liver uptake of ¹⁸F-DMFB decreased from 10 to 120 min and showed fast clearance (10, 30, 60, and 120 min: 11.19, 5.7, 2.47, and 0.4 percentage injected dose/g). Furthermore, ¹⁸F-DMFB allowed visualization of metastatic lesions immediately after injection and was retained in lesions for over 60 min, with a high tumor-to-background ratio. Conclusion: ¹⁸F-DMFB demonstrated a high melanin-targeting ability and tumor-specific tumor uptake in both primary and metastatic lesions in animal models bearing malignant melanoma. ¹⁸F-DMFB may be a potential PET imaging agent for melanoma.

Targeted radiotherapy of pigmented melanoma with 131I-5-IPN

Purpose

There has been no satisfactory treatment for advanced melanoma until now. Targeted radionuclide therapy (TRNT) may be a promising option for this heretofore lethal disease. Our goal in this study was to synthesize ¹³¹I-N-(2-(diethylamino)ethyl)-5-(iodo-131I)picolinamide (¹³¹I-5-IPN) and evaluate its therapeutic ability and toxicity as a radioiodinated melanin-targeting therapeutic agent.

Methods

The trimethylstannyl precursor was synthesized and labeled with ¹³¹I to obtain ¹³¹I-5-IPN. The pharmacokinetics of ¹³¹I-5-IPN was evaluated through SPECT imaging, and its biodistribution was assessed in B16F10 tumor models and in A375 human-to-mouse xenografts. For TRNT, B16F10 melanoma-bearing mice were randomly allocated to receive one of five treatments (n = 10 per group): group A (the control group) received 0.1 mL saline; group B was treated with an equimolar dose of unlabeled precursor; group C received 18.5 MBq of [¹³¹I]NaI; group D and E received one or two dose of 18.5 MBq ¹³¹I-5-IPN, respectively. TRNT efficacy was evaluated through tumor volume measurement and biology study. The toxic effects of ¹³¹I-5-IPN on vital organs were assessed with laboratory tests and histopathological examination. The radiation absorbed dose to vital organs was estimated based on biodistribution data.

Results

¹³¹I-5-IPN was successfully prepared with a good radiochemistry yield (55% ± 5%, n = 5), and it exhibited a high uptake ratio in melanin-positive B16F10 cells which indicating high specificity. SPECT imaging and biodistribution of ¹³¹I-5-IPN showed lasting high tumor uptake in pigmented B16F10 models for 72 h. TRNT with ¹³¹I-5-IPN led to a significant anti-tumor effect and Groups D and E displayed an extended median survival compared to groups A, B, and C. The highest absorbed dose to a vital organ was 0.25 mSv/MBq to the liver; no obvious injury to the liver or kidneys was observed during treatment. ¹³¹I-5-IPN treatment was associated with reduction of expression of proliferating cell nuclear antigen (PCNA) and Ki67 and cell cycle blockage in G2/M phase in tumor tissues. Decreased vascular endothelial growth factor and CD31 expression, implying reduced tumor growth, was noted after TRNT.

Conclusion

We successfully synthesized ¹³¹I-5-IPN, which presents long-time retention in melanotic melanoma. TRNT with ¹³¹I-5-IPN has the potential to be a safe and effective strategy for management of pigmented melanoma.

Multimodality reporter gene imaging: Construction strategies and application

Molecular imaging has played an important role in the noninvasive exploration of multiple biological processes. Reporter gene imaging is a key part of molecular imaging. By combining with a reporter probe, a reporter protein can induce the accumulation of specific signals that are detectable by an imaging device to provide indirect information of reporter gene expression in living subjects. There are many types of reporter genes and each corresponding imaging technique has its own advantages and drawbacks. Fused reporter genes or single reporter genes with products detectable by multiple imaging modalities can compensate for the disadvantages and potentiate the advantages of each modality. Reporter gene multimodality imaging could be applied to trace implanted cells, monitor gene therapy, assess endogenous molecular events, screen drugs, etc. Although several types of multimodality imaging apparatus and multimodality reporter genes are available, more sophisticated detectors and multimodality reporter gene systems are needed.

PET and SPECT imaging of melanoma: the state of the art

Melanoma represents the most aggressive form of skin cancer, and its incidence continues to rise worldwide. 18F-FDG PET imaging has transformed diagnostic nuclear medicine and has become an essential component in the management of melanoma, but still has its drawbacks. With the rapid growth in the field of nuclear medicine and molecular imaging, a variety of promising probes that enable early diagnosis and detection of melanoma have been developed. The substantial preclinical success of melanin- and peptide-based probes has recently resulted in the translation of several radiotracers to clinical settings for noninvasive imaging and treatment of melanoma in humans. In this review, we focus on the latest developments in radiolabeled molecular imaging probes for melanoma in preclinical and clinical settings, and discuss the challenges and opportunities for future development.

Rapid radiosynthesis of two [18F]-labeled nicotinamide derivatives for malignant melanoma imaging

The rapid synthesis of two radiofluoronicotinamide derivatives, namely, [¹⁸F]MEL050 and [¹⁸F]MEL-2F has been simply performed starting from commercial materials. [¹⁸F]MEL-2F is a new, potential analogue PET-probe for melanoma imaging. [¹⁸F]MEL050 is already an excellent PET imaging probe for early specific diagnosis. The synthesis involves coupling step to obtain the precursor followed by radiofluorination. During the synthesis of the precursors different coupling reagents, such as HBTU, TFFH, HOBT, COMU and PyCIU have been applied. PyClU was found the best to reduce the coupling period to < 1h. The labeled compounds were isolated and purified by HPLC. In the in-vitro study three kinds of cells, namely, Melur (melanin free), KB-3 carcinoma cell line (non-melanoma) and B16-F10 melanoma cell line were used to evaluate the uptake of the radiotracers.

Design, synthesis and evaluation of antiproliferative activity of melanoma-targeted histone deacetylase inhibitors

The clinical validation of histone deacetylase inhibition as a cancer therapeutic modality has stimulated interest in the development of new generation of potent and tumor selective histone deacetylase inhibitors (HDACi). With the goal of selective delivery of the HDACi to melanoma cells, we incorporated the benzamide, a high affinity melanin-binding template, into the design of HDACi to generate a new series of compounds 10a-b and 11a-b which display high potency towards HDAC1 and HDAC6. However, these compounds have attenuated antiproliferative activities relative to the untargeted HDACi. An alternative strategy furnished compound 14, a prodrug bearing the benzamide template linked via a labile bond to a hydroxamate-based HDACi. This pro-drug compound showed promising antiproliferative activity and warrant further study.

Synthesis, radiolabelling, and biodistribution studies of triazole derivatives for targeting melanoma

Molecular probes that target specific markers expressed in solid tumours are in demand for cancer imaging and radionuclide therapy applications. The synthesis, characterization, and in vivo evaluation of radioiodinated triazoles designed as probes to target melanoma are described here. Compounds were prepared using a thermal click reaction between ethynylstannane and methyl 2-azidoacetate, resulting in preferential formation of the corresponding 1,4-tin triazole. The primary amine of various targeting vectors was then coupled to the resulting tin triazole methyl ester. These precursors were labelled with no carrier added 123I or 125I and purified by high performance liquid chromatography to give isolated radiochemical yields between 6% and 51% and radiochemical purities of >95% in all cases. Among the evaluated compounds, N-(2-diethylamino-ethyl)-2-(4-iodo-[1,2,3]triazol-1-yl)acetamide (7a) and N-(1-benzylpiperidin-4-yl)-2-(4-iodo-1H-1,2,3-triazol-1-yl)acetamide (7d) showed the most promising in vivo data, and their 123I-labelled forms were used in single photon emission computed tomography computed tomography (SPECT–CT) imaging studies. The imaging data showed excellent tumour visualization with a very high signal to noise ratio.

Synthesis of meta-substituted [(18)F]3-fluoro-4-aminopyridine via direct radiofluorination of pyridine N-oxides

Due to their electron-rich aromatic structure, nucleophilic (radio)fluorination of pyridines is challenging, especially at the meta position. In this paper, we describe the first example of direct fluorination of a pyridine N-oxide to produce a meta fluorinated pyridine. Specifically, fluorination of 3-bromo-4-nitropyridine N-oxide produced in several minutes 3-fluoro-4-nitropyridine N-oxide in moderate yield at room temperature. This intermediate compound was later converted to 3-fluoro-4-aminopyridine easily by catalytic hydrogenation. Furthermore, this approach was successfully applied for labeling with fluorine-18. The use of pyridine N-oxides for the preparation of fluoropyridines is unprecedented in the chemical literature and has the potential to offer a new way for the synthesis of these important structures in pharmaceuticals and radiopharmaceuticals.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

TYR as a multifunctional reporter gene regulated by the Tet-on system for multimodality imaging: an in vitro study

The human tyrosinase gene TYR is a multifunctional reporter gene with potential use in photoacoustic imaging (PAI), positron emission tomography (PET), and magnetic resonance imaging (MRI). We sought to establish and evaluate a reporter gene system using TYR under the control of the Tet-on gene expression system (gene expression induced by doxycycline [Dox]) as a multimodality imaging agent. We transfected TYR into human breast cancer cells (MDA-MB-231), naming the resulting cell line 231-TYR. Using non-transfected MDA-MB-231 cells as a control, we verified successful expression of TYR by 231-TYR after incubation with Dox using western blot, cellular tyrosinase activity, Masson-Fontana silver staining, and a cell immunofluorescence study, while the control cells and 231-TYR cells without Dox exposure revealed no TYR expression. Detected by its absorbance at 405 nm, increasing concentrations of melanin correlated positively with Dox concentration and incubation time. TYR expression by Dox-induced transfected cells shortened MRI T1 and T2 relaxation times. Photoacoustic signals were easily detected in these cells. (18)F-5-fluoro-N-(2-[diethylamino]ethyl)picolinamide ((18)F-5-FPN), which targets melanin, quickly accumulated in Dox-induced 231-TYR cells. These show that TYR induction of melanin production is regulated by the Tet-on system, and TYR-containing indicator cells may have utility in multimodality imaging.

Imaging malignant melanoma with (18)F-5-FPN

PURPOSE

Radiolabelled benzamides are attractive candidates for targeting melanoma because they bind to melanin and exhibit high tumour uptake and retention. (18)F-5-Fluoro-N-(2-[diethylamino]ethyl)picolinamide ((18)F-5-FPN), a benzamide analogue, was prepared and its pharmacokinetics and binding affinity evaluated both in vitro and in vivo to assess its clinical potential in the diagnosis and staging of melanoma.

METHODS

(18)F-5-FPN was prepared and purified. Its binding specificity was measured in vitro in two different melanoma cell lines, one pigmented (B16F10 cells) and one nonpigmented (A375m cells), and in vivo in mice xenografted with the same cell lines. Dynamic and static PET images using (18)F-5-FPN were obtained in the tumour-bearing mice, and the static images were also compared with those acquired with (18)F-FDG. PET imaging with (18)F-5-FPN was also performed in B16F10 tumour-bearing mice with lung metastases.

RESULTS

(18)F-5-FPN was successfully prepared with radiochemical yields of 5 - 10 %. Binding of (18)F-5-FPN to B16F10 cells was much higher than to A375m cells. On dynamic PET imaging B16F10 tumours were visible about 1 min after injection of the tracer, and the uptake gradually increased over time. (18)F-5-FPN was rapidly excreted via the kidneys. B16F10 tumours were clearly visible on static images acquired 1 and 2 h after injection, with high uptake values of 24.34 ± 6.32 %ID/g and 16.63 ± 5.41 %ID/g, respectively, in the biodistribution study (five mice). However, there was no visible uptake by A375m tumours. (18)F-5-FPN and (18)F-FDG PET imaging were compared in B16F10 tumour xenografts, and the tumour-to-background ratio of (18)F-5-FPN was ten times higher than that of (18)F-FDG (35.22 ± 7.02 vs. 3.29 ± 0.53, five mice). (18)F-5-FPN PET imaging also detected simulated lung metastases measuring 1 - 2 mm.

CONCLUSION

(18)F-5-FPN specifically targeted melanin in vitro and in vivo with high retention and affinity and favourable pharmacokinetics. (18)F-5-FPN may be an ideal molecular probe for melanoma diagnosis and staging.

Vectors for the delivery of radiopharmaceuticals in cancer therapeutics

Internal radiation using radiopharmaceuticals promises efficient cancer therapeutics. The specificity and selectivity required for screening and pinpointing tumor cells for cell-kill has been made possible by targeted ligands based on 'magic bullet' and tracer principle- theories nearing a century. Overexpression of certain receptors has been exploited using biomolecules for targeting. The pragmatic analysis, however, is not as promising compared with the theoretical knowledge of available gamut of vectors and targets. The complex interplay of in vitro and in vivo parameters, and the effect of radionuclides involve a systematic assessment of radiopharmaceuticals as diagnostic and therapeutic agent. This review presents different vectors with their pros and cons, present status and recent design variations followed by a future perspective based on novel approaches.

Fluorine-18 radiochemistry, labeling strategies and synthetic routes

Fluorine-18 is the most frequently used radioisotope in positron emission tomography (PET) radiopharmaceuticals in both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.7 min half-life, 635 keV positron energy), along with high specific activity and ease of large scale production, make it an attractive nuclide for radiochemical labeling and molecular imaging. Versatile chemistry including nucleophilic and electrophilic substitutions allows direct or indirect introduction of (18)F into molecules of interest. The significant increase in (18)F radiotracers for PET imaging accentuates the need for simple and efficient (18)F-labeling procedures. In this review, we will describe the current radiosynthesis routes and strategies for (18)F labeling of small molecules and biomolecules.

Melanoma and nuclear medicine

Supported by a large body of published work, the contribution of nuclear medicine technologies to the assessment of melanoma has been increasing in recent years. Lymphoscintigraphy-assisted sentinel lymph node biopsy and PET are in continuous evolution with the aid of technological imaging advances, making it possible to fuse functional and anatomic images (e.g., with SPECT/CT, PET/CT and 3D rendering systems). The development of hybrid fluorescent-radioactive tracers that enable high-quality preoperative lymphoscintigraphy and SPECT/CT, and the optimization of modern intraoperative portable imaging technologies, such as free-hand SPECT and portable γ-cameras, are important innovations that have improved sentinel lymph node identification in complex anatomical areas, such as the pelvis and head and neck. Concurrently, ¹⁸F-fluorodeoxyglucose-PET has proved its usefulness in the clinical staging and treatment decision-making process, and there is also emerging evidence regarding its utility in the evaluation of therapeutic response. The potential uses of other novel PET radiotracers could open up a new field of use for this technique. In this article, we review the current and future role of nuclear medicine in the management of melanoma.

Science to practice: Cellular therapy of Parkinson disease--a new radiotracer to target transplanted dopaminergic cells with PET

The article by Bu and colleagues (1) introduces N-[2-(diethylamino)ethyl]-(18)F-5-fluoropicolinamide ((18)F-P3BZA) as a promising positron emission tomography (PET) radiotracer to target transplanted porcine retinal pigment epithelial (pRPE) cells in the striatum. This strategy to monitor cellular therapy of Parkinson disease has a high translational potential and in the future may help us to better understand some of the controversial results reported in clinical trials.

Tyrosinase as a multifunctional reporter gene for Photoacoustic/MRI/PET triple modality molecular imaging

Development of reporter genes for multimodality molecular imaging is highly important. In contrast to the conventional strategies which have focused on fusing several reporter genes together to serve as multimodal reporters, human tyrosinase (TYR)--the key enzyme in melanin production--was evaluated in this study as a stand-alone reporter gene for in vitro and in vivo photoacoustic imaging (PAI), magnetic resonance imaging (MRI) and positron emission tomography (PET). Human breast cancer cells MCF-7 transfected with a plasmid that encodes TYR (named as MCF-7-TYR) and non-transfected MCF-7 cells were used as positive and negative controls, respectively. Melanin targeted N-(2-(diethylamino)ethyl)-18F-5-fluoropicolinamide was used as a PET reporter probe. In vivo PAI/MRI/PET imaging studies showed that MCF-7-TYR tumors achieved significant higher signals and tumor-to-background contrasts than those of MCF-7 tumor. Our study demonstrates that TYR gene can be utilized as a multifunctional reporter gene for PAI/MRI/PET both in vitro and in vivo.

Intrastriatal transplantation of retinal pigment epithelial cells for the treatment of Parkinson disease: in vivo longitudinal molecular imaging with 18F-P3BZA PET/CT

PURPOSE

To evaluate the performance of N-[2-(diethylamino)ethyl]-(18)F-5-fluoropicolinamide ((18)F-P3BZA) for visualizing porcine retinal pigment epithelium (pRPE) cells transplanted in the striatum for the treatment of Parkinson disease and to monitor the long-term activity of implanted pRPE cells by means of (18)F-P3BZA positron emission tomography (PET)/computed tomography (CT) in vivo.

MATERIALS AND METHODS

Animal work was conducted in accordance with the administrative panel on laboratory animal care. In vitro cell uptake of (18)F-P3BZA was determined with incubation of melanotic pRPE or amelanotic ARPE-19 cells with (18)F-P3BZA. To visualize the implanted pRPE cells in vivo, normal rats (four per group) were injected with pRPE or ARPE-19 cells attached to gelatin microcarriers in the left striatum and with control gelatin microcarriers in the right striatum and followed up with small animal PET/CT. Longitudinal PET/CT scans were acquired in 12 rats up to 16 days after surgery. Postmortem analysis, which included autoradiography and hematoxylin-eosin, Fontana-Masson, and immunofluorescence staining, was performed. Data were compared with the Student t test, analysis of variance, and regression analysis.

RESULTS

(18)F-P3BZA accumulated in pRPE cells effectively (3.48% of the injected dose [ID] per gram of brain tissue ± 0.58 at 1 hour after injection of the probe at 2 days after surgery in vivo) but not in control ARPE-19 cells (P < .05). Longitudinal PET/CT scans revealed that the activity of implanted pRPE cells decreased over time, as evidenced by a reduction in (18)F-P3BZA uptake (3.39% ID/g ± 0.18, 2.49% ID/g ± 0.41, and 1.20% ID/g ± 0.13 at days 2, 9, and 16, respectively; P < .05). Postmortem analysis helped confirm the results of in vivo imaging.

CONCLUSION

(18)F-P3BZA PET/CT is a feasible technique for visualizing and detecting the activity of implanted RPE cells in vivo.

Theranostics of malignant melanoma with 64CuCl2

Human copper transporter 1 (CTR1) is overexpressed in a variety of cancers. This study aimed to evaluate the use of (64)CuCl2 as a theranostic agent for PET and radionuclide therapy of malignant melanoma.

METHODS

CTR1 expression levels were detected by Western blot analysis of a group of tumor cell lines. Two melanoma cell lines (B16F10 and A375M) that highly expressed CTR1 were then selected to study the uptake and efflux of (64)CuCl2. Mice bearing B16F10 or A375M tumors (n = 4 for each group) were subjected to 5 min of static whole-body PET scans at different time points after intravenous injection of (64)CuCl2. Dynamic scans were also obtained for B16F10 tumor-bearing mice. All mice were sacrificed at 72 h after injection of (64)CuCl2, and biodistribution studies were performed. Mice bearing B16F10 or A375M tumors were further subjected to (64)CuCl2 radionuclide therapy. Specifically, when the tumor size reached 0.5-0.8 cm in diameter, tumor-bearing mice were systemically administered (64)CuCl2 (74 MBq) or phosphate-buffered saline, and tumor sizes were monitored over the treatment period.

RESULTS

CTR1 was found to be overexpressed in the cancer cell lines tested at different levels, and high expression levels in melanoma cells and tissues were observed (melanotic B16F10 and amelanotic A375M). (64)CuCl2 displayed high and specific uptake in B16F10 and A375M cells. In vivo (64)CuCl2 PET imaging demonstrated that both B16F10 and A375M tumors were clearly visualized. Radionuclide treatment studies showed that the tumor growth in both the B16F10 and the A375M models under (64)CuCl2 treatment were much slower than that of the control group.

CONCLUSION

Both melanotic and amelanotic melanomas (B16F10 and A375M) tested were found to overexpress CTR1. The tumors can be successfully visualized by (64)CuCl2 PET and further treated by (64)CuCl2, highlighting the high potential of using (64)CuCl2 as a theranostic agent for the management of melanoma.

A novel aliphatic 18F-labeled probe for PET imaging of melanoma

Radiofluorinated benzamide and nicotinamide analogues are promising molecular probes for the positron emission tomography (PET) imaging of melanoma. Compounds containing aromatic (benzene or pyridine) and N,N-diethylethylenediamine groups have been successfully used for development of melanin targeted PET and single-photon emission computed tomography (SPECT) imaging agents for melanoma. The objective of this study was to determine the feasibility of using aliphatic compounds as a molecular platform for the development of a new generation of PET probes for melanoma detection. An aliphatic N,N-diethylethylenediamine precursor was directly coupled to a radiofluorination synthon, p-nitrophenyl 2-(18)F-fluoropropionate ((18)F-NFP), to produce the probe N-(2-(diethylamino)ethyl)-2-(18)F-fluoropropanamide ((18)F-FPDA). The melanoma-targeting ability of (18)F-FPDA was further evaluated both in vitro and in vivo through cell uptake assays, biodistribution studies, and small animal PET imaging in C57BL/6 mice bearing B16F10 murine melanoma tumors. Beginning with the precursor (18)F-NFP, the total preparation time for (18)F-FPDA, including the final high-performance liquid chromatography purification step, was approximately 30 min, with a decay-corrected radiochemical yield of 79.8%. The melanin-targeting specificity of (18)F-FPDA was demonstrated by significantly different uptake rates in tyrosine-treated and untreated B16F10 cells in vitro. The tumor uptake of (18)F-FPDA in vivo reached 2.65 ± 0.48 %ID/g at 2 h postinjection (p.i.) in pigment-enriched B16F10 xenografts, whereas the tumor uptake of (18)F-FPDA was close to the background levels, with rates of only 0.37 ± 0.07 %ID/g at 2 h p.i. in the nonpigmented U87MG tumor mouse model. Furthermore, small animal PET imaging studies revealed that (18)F-FPDA specifically targeted the melanotic B16F10 tumor, yielding a tumor-to-muscle ratio of approximately 4:1 at 1 h p.i. and 7:1 at 2 h p.i. In summary, we report the development of a novel (18)F-labeled aliphatic compound for melanoma imaging that can be easily synthesized in high yields using the radiosynthon (18)F-NFP. The PET probe (18)F-FPDA exhibits high B16F10 tumor-targeting efficacy and favorable in vivo pharmacokinetics. Our study demonstrates that aliphatic compounds can be used as a new generation molecular platform for the development of novel melanoma targeting agents. Further evaluation and optimization of (18)F-FPDA for melanin targeted molecular imaging are therefore warranted.